112 bulletin: museum of comparative zoology. 



"centrosome" of writers on spermatid metamorphosis. I have been, 

 unable to trace any connection between this body and the centrosome 

 of the last maturation di\dsion since, as in previous divisions, the 

 centrosome disappears during the anaphase. During the early stages 

 of the metamorphosis granules staining with hematoxylin are frequently 

 seen in the cytoplasm, and in some cases may be applied to the nuclear 

 membrane. There may be several of these granules in a single sper- 

 matid, and it is, of course, impossible to determine whether any of 

 them have any connection with the centrosome. Extending out from 

 the centrosome into the elongated portion of the cell is a fine fibril, 

 the axial filament. At this stage the axial filament is well defined for 

 a short distance from the centrosome and then gradually becomes 

 thinner until it disappears altogether. The spermatid continues to 

 elongate (Fig. 216) while the centrosome and axial filament increase 

 in size. The Nebenkern becomes much elongated and travels out 

 along the axial filament, part of it being continually left behind to 

 form a distinct envelope about the filament. Figure 218 is a later 

 stage, in which the entire Nebenkern has been converted into an 

 envelope surrounding the axial filament. " The nucleus lies at one end 

 of the greatly elongated spermatid and is surrounded by a very thin 

 layer of cytoplasm. 



From now on marked changes take place in both cytoplasm and 

 nucleus. The latter becomes smaller and conical, and stains a nearly 

 uniform gray with here and there minute deeply staining granules 

 (Fig. 219). Figures 220-222 represent successively later stages in the 

 elongation of the nucleus and centrosome. At this time, especially 

 when imperfectly fixed, the nucleus usually shows a very finely fibrillar 

 structure. There is practically no cytoplasm surrounding the nucleus, 

 although it is still plainly enclosed by the cell membrane. Figure 223 

 shows the anterior end of a nearly mature spermatozoon. The nu- 

 cleus, which again stains deeply with hematoxylin, has become greatly 

 elongated to form the head of the spermatozoon and at the anterior 

 end tapers to a fine point; but there is nothing which can be consid- 

 ered an acrosome. The nucleus is still surrounded by the cell wall, 

 but there is apparently no cytoplasm in this region and the cell wall 

 is probably lost a little later. The centrosome is much elongated and 

 forms the so-called middle piece, while the tail is composed of a central 

 deeply staining fiber, formed from the axial filament, and an envelope 

 derived from the Nebenkern. the two at this stage being indistinguish- 

 able. Surrounding the central fiber and forming the greater part of 

 the tail is a lighter envelope derived from the cytoplasm of the sperma- 



